Chemical Connections to Climate Change

Tom Kuntzleman | Wed, 01/25/2017 - 11:11

Chemical Connections to Climate Change

Did you know that climate change can be used as a backdrop to discuss several ideas in chemistry? In this post I’ll describe how several chemical concepts including Beer’s Law, the chemical composition of the atmosphere, combustion chemistry, and solutions can all be used in a discussion of how CO2 keeps our planet warm – and how excess CO2 warms it even more.

Composition of the atmosphere

We’ll start by treating our atmosphere as a solution. Several different gases make up our atmosphere (Table 1). Because N2(g) is the compound present in greatest amount in the atmosphere, N2 is the “solvent” in our atmospheric “solution”. The solute present in largest amount is O2 (g), but certainly there are many others.

Table 1: Gases present in the atmosphere

Gas

Name

Percent by volume

N2

nitrogen

78%

O2

oxygen

21%

Ar

argon

1%

H2O

gaseous water

0 to 4%

CO2

carbon dioxide

0.04% and increasing

The greenhouse effect

Some of these solute gases warm our planet by absorbing infrared (IR) light radiation that would otherwise escape into space. Without this warming effect (called the greenhouse effect), the average temperature on Earth would be a chilly 255 K (that’s -18 oC, or the temperature on an extremely cold winter day). Because solute gases in our atmosphere absorb IR light, Earth’s average temperature is 288 K (that’s 15 oC, about the temperature on a slightly cool spring day). Thus Earth is 33 K warmer than what would be expected if there was no atmosphere.1

While both N2(g) and O2(g) are present in very large amounts in the atmosphere, these gases do not absorb IR light and therefore do not contribute to the greenhouse effect. On the other hand, CO2(g) and H2O(g) do absorb IR light and upon doing so gain energy which is transferred to the rest of the Earth. Even though these gases are present in small amounts, they are very good at absorbing infrared light. Thus, these atmospheric gases are the main contributors to the greenhouse effect. Of the 33 K increase in temperature due to the greenhouse effect, H2O(g) contributes half (16.5 K) and CO2 contributes 20% (6.6 K). Clouds (25%) and other trace gases (5%) make up the remainder.1

Connection between climate change and Beer’s Law

Large amounts of CO2(g) are regularly pumped into the atmosphere though various combustion reactions, such as that found during the burning of isooctane (the main component in gasoline):

2 C8H18 (g) + 25 O2 (g)à 16 CO2 (g) + 18 H2O (g) Equation 1

Climatologists tell us that the resulting increase in atmospheric CO2(g) concentration has the effect of increasing the average temperature on Earth. So let’s ask the question: How much hotter would we expect the average temperature of Earth to rise upon addition of more CO2(g) into the atmosphere?2

We’ll gain some insight into this question using Beer’s Law, which describes the amount of light absorbed by a compound in a solution:

A = ebc Equation 2

Where A is the amount of light absorbed, e is the molar absorptivity of the compound (a measure of how well the compound absorbs the light), b is the path length through which the light travels, and c is the concentration of the compound in the solution. In our case, A is the amount of IR light absorbed by CO2(g) in the atmosphere, e is how well CO2(g) absorbs IR light, b would be the thickness of Earth’s atmosphere, and c is the concentration of CO2(g) in the atmosphere.

Back in 1850, the concentration of CO2(g) in the atmosphere was 285 ppm.3 Today, the concentration of CO2(g) is over 400 ppm.3,4 Let’s use these values and Beer’s Law to estimate the ratio of IR light absorbed by CO2(g) in 1850 (A1850) vs. today (Atoday):

Based on Beer’s Law, we’d expect CO2(g) to be absorbing about 40% more IR light than it did back in 1850, due to this increase in concentration.

Let’s find the expected global rise in temperature due to this 40% increase in light absorbed by additional CO2(g). The baseline contribution of CO2(g) to the greenhouse effect is 6.6 K (see above). Let’s set this value equal to the contribution of CO2(g) to the greenhouse effect in 1850. Multiplying this value by 1.4 gives: 6.6 K x 1.4 = 9.2 K. Thus we would predict that the additional CO2 in the atmosphere would increase the contribution of CO2(g) to the greenhouse effect from 6.6 K to 9.2 K. That’s a change of +2.6 K (9.2 K – 6.6 K). Indeed, average global temperatures have risen 1.0 K since 1850.5 Our estimate is in right the ballpark, but well over two times too high. Surely this is because there are many more factors on the global scale that our simple approach has not taken into account. Nevertheless, we can at least gain some insight into how increased CO2(g) concentrations in our atmosphere have caused the average global temperatures to rise…and we can do so using chemistry!

Do you ever bring up climate change or global warming in your classes? If so, how do you approach the issue? What kinds of chemical concepts do you use to discuss these issues? We would love to hear from you, so be sure to let us know what you think.

Noticing that combustion reactions release water (Equation 1), it is natural to ask why no one is concerned about increases in global temperature due to increases in atmospheric water vapor. The answer is that water does not “build up” in the atmosphere the way CO2 does (and some other gases do). When “too much” water vapor builds up in the atmosphere, it rains, removing the excess water.

Comments 7

Enjoyed reading the connections made here. Never thought about looking at climate change through the lens of Beer's Law. This post made me think of another chemistry connection. Using LeChatelier's principle to account for increased acidity of our oceans. Focus on the equilibrium between CO2 in air and dissolved CO2, forming carbonic acid, and how LeChatelier can account for favoring the dissolution of CO2 when we increase amount of CO2 in atmosphere.

I actually take the opposite approach and bring chemistry into environmental science. It usually goes something like this: when talking about greenhouse gases, water is often left out of the equation - yet it is much more abundant and contributes far more to the greenhouse effect. So why don't we hear about much water in the atmosphere? We discuss how humidity changes between summer and winter and relate it to weather patterns - rising temperatures mean more evaporation and falling temperatures can lead to preciptiation. More severe storms are one of the strongest evidences of climate change. You could get more technical with it if you wanted to get into gas laws and vaporization pressures.

By the way, did you know we can measure CO2 levels from the 1850s today? Bubbles of atmospheric gas get trapped into the crystal structure of ice as it forms. With the yearly melt/freeze cycles, glacial ice forms layers much like tree rings that can be counted and dated. With a deep enough ice core we can sample the air pockets much as you describe here to get our data.

Hi John. It's great to hear your take on how you approach this subject. Do you teach environmental science and not chemistry? Or do you teach both? As you note, the subject of climate change is incredibly rich with chemistry! Thank you for the resource you provided from the American Chemical Society regarding water vapor in the atmosphere. I agree with you that the study of ice cores is incredibly interesting. I especially enjoy learning about how isotopes are used to estimate the temperature in times past. I've considered incorporating a discussion of such in my introductory chemistry classes, but I have yet to do so.

I do teach both classes, as well as Materials Science. It's not that I don't mention climate change in chemistry - I definitely do, especially when we talk about gas laws and thermodynamics, just not something I think about including often enough.

Increased carbon dioxide in the atmosphere due to the burning of fossil fuels has implications for ocean life. Excess atmospheric carbon dioxide dissolves in oceans, causing the pH of oceans to drop. This has implications for marine life that depends upon calcium carbonate. Below you can view a short video that illustrates some of the chemistry behind how this occurs.